The present invention relates generally to the field of removal or stripping of photodefinable materials from a substrate used in the manufacture of electronic devices. In particular, the present invention is related to the removal of patterned photoresist and patterned photoimageable dielectric materials from such substrates.
In very general terms, the fabrication of integrated circuits involves steps for producing polished silicon wafers, steps for imaging integrated circuit pattern geometries on the various wafer surfaces, steps for generating the desired pattern on the wafer, and steps for producing dielectric layers on the wafer.
The imaging process involves the use of photoresists applied to the wafer surface. Photoresists are compositions which undergo change in response to light of particular wavelength such that selective exposure of the compositions through a suitable patterned mask, followed by development to remove exposed or non-exposed portions of the photoresist, depending upon whether the resist is positive- or negative-tone, leaves on the surface of the wafer a pattern of resist which replicates either the positive or negative of the mask pattern, and which thus permits subsequent processing steps to be performed to the desired selective pattern.
In the manufacture of electronic devices using wafers, the photoresists are typically liquid compositions of organic light-sensitive materials which are either polymers or are used in conjunction with polymers, dissolved in an organic solvent. Critical to the effectiveness of the selective light exposure and development in forming a resist pattern on the wafer substrate is the initial application of the photoresist composition in a thin layer of essentially uniform thickness on the wafer surface. Typically, such photoresist compositions are applied by spin-coating. In such spin-coating process, a predetermined amount of photoresist composition is disposed on the wafer surface and the wafer is then subjected to rotation, such as at about 500 to about 600 rpm, to cause the photoresist composition to spread out evenly as a layer along the wafer surface and such that excess photoresist is spun off the edges of the wafer.
Despite its widespread use, spin-coating has certain problems. Due to the surface tension of the photoresist composition, some of the resist may wick around to and coat the backside of the wafer during the spin-coating process. Another problem is that as the spin-coating process progresses, the photoresist becomes progressively more viscous as solvent evaporates from the resist composition and in the later stages of the process the resist can leave whiskers of resist which dry on the wafer edge. As the resist continues to dry and increases in viscosity, excess resist is less likely to leave the wafer and instead builds up as an edge bead at the outer edge of the wafer. Such edge beads are typically thicker than the photoresist layer which creates problems for close proximity or contact printing.
Such spin-coating problems cause significant difficulties in overall integrated circuit fabrication processes. Resist on the backside of the wafer can be deposited elsewhere and cause contamination, and also prevents the wafer from lying flat on ultraflat surfaces, thereby affecting focus, alignment, planarity and the like, in subsequent processing steps. Whiskers on the wafer edges can easily break off in subsequent processing steps and cause particulate contamination in virtually all of the manufacturing equipment. Edge beads lead to a distorted surface which can greatly affect focus, alignment, planarity and the like. Edge beads may also crumble and cause particulate contamination.
Conventional methods for removing such residual photoresist on the wafer edge typically include the step of dispensing a stream of solvent at the edge of the wafer to dissolve the photoresist residue. Such solvent stream may be applied to the front side, or back side of the wafer and then wick around to the front side by capillary action, or may be applied to both the front and back sides. A variety of solvents are known for removing photoresist edge beads, such as ethyl lactate and propylene glycol monomethyl ether acetate. For example, U.S. Pat. No. 5,814,433 (Nelson et al.) discloses a mixture of ethyl lactate and N-methylpyrrolidone. Japanese Unexamined Patent Application JP 57 162 336 (Kanazawa) discloses a method for coating a resist onto a wafer. In this patent application, resist edge beads are removed by dissolving the resist in n-butyl acetate. Edge beads are removed prior to patterning of the photoresist. No other uses of these solvents are disclosed in the above references.
Many current processes for manufacturing electronic devices use a photoresist layer atop an underlying organic film such as an antireflective coating, a cross-linked bottom layer of a bi-layer resist, an organic aperture filling material, a lift-off-layer, an organic memory layer and the like. It is often desirable to remove the photoresist layer after it has been patterned without removing the underlying organic film, such as during a rework process. Conventional strippers not only remove the photoresist layer, but also remove or damage the underlying organic film. Such damage includes swelling of the underlying organic film, changes in the surface acidity of the film, or both, among other problems. Accordingly, rework procedures using conventional photoresist strippers are impractical or else require removal of the underlying organic film as well and subsequent reapplication of both the underlying organic film and photoresist top coat. Such rework procedures are costly and time consuming, typically resulting in the discarding of wafers that could otherwise be reused.
There is a need for a method of removing a patterned photoresist layer from an underlying organic film, without removing or damaging such underlying organic film.
U.S. Pat. No. 6,270,949 B1 (Faure et al.) discloses the use of certain alkyl esters of acetic acid as single component developers for photoresists. This patent fails to disclose the use of such solvents to selectively remove a patterned photoresist from an underlying organic film.
U.S. Pat. No. 5,637,436 (Johnson) discloses a mixture of an alkylacetate ester with an alkyl alcohol for use in removing a photoresist edgehead. This patent fails to disclose the use of an alkyl acetate ester to selectively remove a patterned photoresist from an underlying organic film.